This invention relates to detection, measurement and control of the formation of adherent precipitates such as scale, paraffin, wax, etc. on various surfaces.
The formation of aherent precipitates on equipment surfaces immersed in liquids is a long standing, widespread and costly problem in the industry. Such deposits reduce the rates of heat transfer, increase corrosion and erosion, clog flow lines and interfere with the proper functioning of instruments and control systems.
The most common form of such troublesome foulant coatings is adherent inorganic scale which often precipitates from water used in industrial equipment. For example, insoluble deposits of alkaline earth metal carbonates and sulfates frequently precipitate on the surfaces of heat exchanger tubes, thus reducing by major amounts the rates of heat transfer. The fact that the tubes are hot is a primary reason for such scale formation.
Although adherent inorganic scale is the most common form of foulant, it is emphasized that adherent organic deposits are also major problems in certain industries. Thus, the formation of harmful precipitates is not confined to aqueous systems. For example, in the refining of oil sticky adherent deposits form on metal surfaces of the reactors, heat exchangers or transfer lines. These deposits are often the result of heating of the oil being processed, which heating changes or decomposes asphaltic constituents, asphaltines or similar substances to form undesired adherent coatings. In other instances cooling, instead of heating, is the cause of the problem. For example, crude petroleum oil will deposit adherent coatings of paraffin wax when the temperature of the oil or of the surfaces over which it passes is lowered sufficiently.
Where a liquid is treated with chemicals to control corrosion, bacteria or other characteristics of the liquid, adherent scale derived from such chemicals may also be formed.
Scale or other deposited foulant coating is also a troublesome occurrence in many systems containing organic liquids. For example, deposits frequently occur in high wattage electrical transformers in which the windings are immersed in hydrocarbons or in halogenated aromatic compounds and the like; in hydraulic oil systems containing polyols, ethers and other organics; in heat transfer liquid systems such as heavy oil, bisphenol A or similar high boiling organics; and in numerous organic chemical processing units.
Scale and other harmful foulant coatings are likewise found in two-phase systems. For example, in the processing of freshly produced crude oil, the fluid is heated in a "heater-treater" unit to separate the unwanted salt water. Alkaline earth metal carbonates and sulfates are often present as adherent scale in such treating systems, the scale being sometimes mixed with various amounts of organic material.
There exists a major need for a practical, commercial method of determining whether or not a system is forming significant scale or other adherent precipitates, of determining the conditions under which scale might form, and of determining the conditions under which such formation can be prevented either by addition of chemical scale inhibitors or by control of process variables. It is highly important that the method be capable of implementation by commercial instruments, which function at all times and which do not require trained chemists or scientists for their operation. It is also extremely important that the method be so sensitive that the propensity of a system to develop scale will be detected without waiting until the foulant has created substantial harm in the commercial system being monitored.
In the past, physical inspection of plant equipment has been the common method of ascertaining the presence and existence of adherent scale and other precipitates. Another common method has been to measure changes in heat transfer rates (or in required liquid flow velocities to maintain a certain heat transfer rate). Both of these common methods suffer from the fatal deficiency that the harm which it is desired to prevent (for example, lowered heat transfer rate) must occur before "preventive" measures can be taken.
Because of the great difficulty of making physical inspections of the industrial equipment itself, one method of making heat exchanger studies is to specially design, construct and operate a laboratory model heat exchanger. Such a model usually includes windows for visual inspection, or includes means for withdrawing heat exchanger tubes so that they can be inspected and analyzed. Similarly, it is known to design laboratory heat exchangers wherein the heat transfer rates are monitored in relation to electrical power input, or steam condensation rates. Obviously, the construction and operation of such laboratory models is expensive and time-consuming and the data obtained with them may not be truly representative of what is occurring in the actual industrial equipment. Furthermore, reliance on changes in heat transfer rates, or on macroscopic inspection of surfaces, produces a fatal insensitivity.
In addition to constructing and operating models of heat exchangers or other industrial equipment, there are frequently employed, in the laboratory, chemical methods related to formation of scale and similar substances. For example, test solutions are prepared which are basically unstable and will, in response to heating or standing, and to the passage of time, yield precipitates of alkaline earth metal carbonates or sulfates. Different chemicals are added to such test solutions, and the degree to which such additives prevent or inhibit precipitation is determined. It is, however, emphasized that such tests do not provide continuous monitoring of an actual commercial system, nor do they necessarily produce significant data relative to formation of adherent scale in the actual system. It is to be noted that adherent scale or other precipitate is extremely harmful, but that those precipitates which are not adherent may be relatively harmless.
Other examples of laboratory procedures relative to scale, etc., involved determining the stability of the water in aqueous systems. Stability is ascertained by measuring or calculating from composition analysis, the minimum amount of acid or base required to effect precipitation. The amount of reagent tolerated by the solution without precipitation is taken as being proportional to stability and thus as being inversely proportional to the scale-forming tendency of the liquid. Such periodic tests can, at best, only be indirectly and uncertainty related to the tendence of an actual system to form adherent scale or other deposits.
In our prior U.S. Pat. Nos. 3,848,187 and 3,951,161, we describe extremely precise high sensitivity methods of employing electrical contact resistance to sense incipient precipitation of a foulant coating such as an adherent scale, paraffin wax or the like. The methods and apparatus described in these patents are useful, effective and of high sensitivity, but require moving parts that could adversely affect operation over long periods of time. Further, moving parts also add complexity and cost.
Detection and measurement of foulant coatings employing variations in heat transfer caused by a buildup of a foulant coating have been known in the past and avoid problems of moving parts. However, all of these methods lack sensitivity required for rapid and real time evaluation and, in addition, are subject to major errors due to various changes that may occur in the fluid during or between measurements.
In one such method, a test surface is heated electrically while monitoring the temperature of its surface that is in contact with the fluid. After a period of immersion in the fluid of which the foulant propensity is to be detected, temperature is again monitored and the temperature difference between the first and second measurements is employed as an indication of the change in foulant coating between the times of the first and second measurements. Prior methods employing this principle of detecting changes in heat transfer characteristics caused by changing foulant coatings, are useful as a practical matter only for detection of large changes in foulant coatings. By the time such a prior art system can provide a useful measurement, serious foulant deposit may have already occurred. Such systems are unable to measure relatively small changes in foulant coatings because the readings vary widely as sensitivity is increased. A problem with such prior systems is the fact that the measured temperature varies with many different parameters of the fluid in which the test surface is immersed. In some systems flow rate through a test cell is increased in order to stabilize cell temperature at the entering fluid temperature. With such high flow velocities, the flow velocity itself becomes most critical. Thus for an instrument of high sensitivity, relatively small variations in any one of a number of parameters of the fluid may cause an output reading to vary from zero to full scale even with only a slight disturbance in a parameter such as flow rate. Fluid parameters that affect this temperature measurement include fluid velocity, viscosity, temperature, composition, thermal conductivity, flow pattern at the surface (which may vary with varying roughness due to increasing foulant coating), and other flow patterns, among others. Therefore, with prior measurements based upon monitoring of changes in heat transfer due to changes in foulant coating, it is necessary to maintain all of these fluid parameters the same at each measuring period so that the fluid at the test surface has the same effect upon surface temperature at one measuring period as it does during a subsequent measuring period. Even under laboratory conditions, such identity of fluid characteristics is exceedingly difficult to achieve. In practical circumstances and in field situations, particularly where an instrument is to be used for long term monitoring of an actual system, control of such fluid characteristics is not feasible.
In summary, previous methods known for monitoring scaling, other than our prior U.S. Pat. Nos. 3,848,187 and 3,951,161, do not detect or measure accumulation of foulant in an actual system before such foulant has built up to a degree sufficient to cause significant damage, nor do such prior systems provide a way to test a particular liquid in order to determine in a relatively short time its foulant propensity.
Accordingly, it is an object of the present invention to detect and/or measure foulant or foulant propensity of a fluid before such foulant will adversely affect operation of a system. Another object of the present invention is the detection and measurement of foulant in a system by means of measurement of heat transfer characteristics and without the necessity of removing a test surface from the fluid in which it is immersed. Another object of the invention is to determine quickly and readily conditions under which foulant of various types will precipitate from various fluids.